In recent years, for reducing the amount of CO2 emission in view of global environmental protection, there is a strong demand for improved fuel efficiency of automobiles. Thus, there is an active movement toward reduction of the vehicle body weight, by improving the strength of vehicle body members while simultaneously reducing the sheet thickness. To this end, high strength steel sheet having TS 590 MPa or more are widely utilized for manufacturing a cold rolled steel sheet and hot-dip galvanized steel sheet, which are formed into vehicle body members by press working. Moreover, in order to ensure satisfactory collision safety characteristics required for automobiles, enhanced absorption of the collision energy is mandatory. For improving the collision-energy absorbing property, an effective measure is to increase the yield ratio. The higher is the yield ratio, the more effectively can the collision energy be absorbed even with a small volume of deformation.
In this regard, as the mechanism for reinforcing the steel sheet to have a tensile strength of 590 MPa or more, it is known to harden ferrite as the matrix phase or utilize a hard phase such as martensite. Among other things, precipitation-strengthened, high strength steel sheet obtained by adding carbide-forming elements, such as Nb, makes it possible readily to improve the yield ratio and reduce the amount of alloying elements that are required for realizing a predetermined strength, thereby lowering the production cost.
However, there is a problem associated with a high strength cold rolled steel sheet of precipitation-strengthened type that, in the annealing step after hot rolling and subsequent cold rolling, precipitates become sparsely coarse thereby causing a large fluctuation in strength or elongation property. Incidentally, the shape fixability significantly deteriorates by high strengthening the steel sheet and reducing its thickness. Thus, press molds are generally designed taking into consideration an estimated deformation amount of the pressed parts upon their removal from the molds after the press formation. In this instance, if the tensile strength of the steel sheet fluctuates considerably, deviation from an estimated deformation amount based on a constant tensile strength becomes large and tends to cause shape defects requiring each member to be corrected one by one by sheet metal working, etc., thereby leading to significantly low mass-production efficiency. Thus, there is demand for minimizing fluctuation especially in strength of the cold rolled steel sheet and hot-dip galvanized steel sheet, i.e., for achieving an excellent material homogeneity.
As noted above, there are demands for minimizing fluctuation in strength and elongation properties of the cold rolled and hot-dip galvanized steel sheets, and improving the cold rolling property. Further, high strength cold rolled steel sheet is significantly affected by the steel sheet structure and precipitated amounts in the hot rolled steel sheet, so that it would be effective to realize a higher strength of the hot rolled steel sheet. In relation to hot rolled steel sheet, Patent Literature 1 (JP 3767132 B2) discloses a method for manufacturing a hot rolled steel sheet having excellent ductility and material homogeneity by controlling the Nb and Ti contents. On the other hand, Patent Literature 2 (JP 2000-212687 A) discloses a hot rolled steel sheet having improved material homogeneity and hole expansion formability by controlling the Ti content.